Introduction
A general strategy for sending digital data reliably through a communications channel of varying quality is to send redundant information so that a stream of transmitted source bits can be recovered without error at a receiver even though the communications channel may be erratic. This is particularly important for one-way broadcasts of audio and multimedia that must be received in real-time with a low error rate. In such cases, a low error rate is achieved partly through the use of forward error correction (FEC) code.
The mobile satellite broadcast channel is such an erratic channel since, particularly at lower elevations angle, the line-of-sight (LOS) between a mobile vehicle and the satellite is often obstructed by trees, buildings, signs, utility poles and wires. Such obstructions attenuate and distort a communications waveform, thereby causing high error rates for brief and longer periods of time. A common approach to reliable satellite broadcasting is to implement spatial diversity by broadcasting duplicate signals from satellites at two different orbital locations. In addition, temporal diversity may also be used by delaying one signal by a fixed amount of time. Indeed, some satellite systems also rely upon terrestrial repeating of the satellite signal which is yet another source of diversity. FIG. 1 illustrates a satellite broadcasting system that has dual diversity from 2 satellites (101 and 102) and is augmented by terrestrial repeating (104), thereby providing 3-fold diversity. The origin of the satellite broadcasts is the hub station (103). Both of the satellites and the terrestrial repeaters broadcast the same source data, but the channels that the data travels over are different so that diversity is provided. A diversity radio in the vehicle (104) would in general receive all the signals (satellite and terrestrial) and use this to reconstruct the source data as faithful as possible based upon the reception from the multiple sources.